This invention relates to a terminal for data communication with contactless-type data carriers, having transceiver electronics with two resonant circuits with an antenna and capacitor for radiating electromagnetic waves with different carrier frequencies.
The data carriers are formed as transponders. After activation by the terminal""s electromagnetic field, data stored in the data carrier can be read out and changed if necessary.
Contactless-type data carriers, for example contactless-type smart cards, are used for a great variety of applications, for example for the use of public transport systems, as electronic purses, health insurance ID cards and the like. Since the number of smart cards that a person carries is in general increasing constantly, it is desirable to have a multifunctional data carrier that can be employed for as many applications as possible.
Higher-frequency transponders are superior to transponders with a lower carrier frequency, in particular with respect to the data transfer rate. On the other hand, high-frequency transponders typically require a large frequency bandwidth in order to have a high data transfer rate. It is therefore desirable to be able to use a high or low carrier frequency alternatively for communication between the terminal and the data carriers depending on the application.
While the choice of carrier frequency used to be largely unregulated, now the carrier frequency, allowable frequency bandwidth and transmitting energy radiated by the terminal are fixed by governing regulations. For example, a carrier frequency of 13.56 MHz is stipulated by the ISO standard.
On the other hand, there are still numerous data carriers that work with another, usually much lower, carrier frequency. For example, for access entitlement to ski lifts one uses data carriers with a carrier frequency of 120-125 kHz integrated into watches, clothes or the like. If communication between access terminal and data carrier shows access entitlement, the access terminal emits an enabling signal, for example to release a blocking apparatus like a turnstile.
To permit terminals to communicate with data carriers of different carrier frequencies, e.g. both with 13.56 MHz data carriers and with 125 kHz data carriers, a known prior art terminal has two resonant circuits, one resonant circuit with a small antenna for low carrier frequency and one resonant circuit with a large antenna for high carrier frequency, see German Patent Document No. DE 198 31 767/U.S. Pat. No. 6,310,581. This involves high additional expenditure and space problems. Moreover, the antennas can influence each other.
German Patent Document No. DE 196 51 719 A1/U.S. Pat. No. 5,813,348 discloses a wireless transceiver system for example for interrogating IC cards which has primary and secondary circuit devices and thus a terminal for communication with contactless-type data carriers. The secondary device includes two resonant circuits having different resonant frequencies. The electromagnetic oscillations to be emitted are radiated via the particular resonant circuit coil.
According to WO 94/03982 A1, two frequencies are used for identification. DE 39 22 977 C2 discloses a trimming circuit wherein electromagnetic oscillations of different frequency positions are radiated via a single antenna, the single resonant circuit being adjusted in its frequency position by connection or disconnection of individual capacitances.
The object of the invention is to substantially reduce the expenditure and the space required for a terminal capable of communicating contactlessly with data carriers over two different data transfer carrier frequencies.
According to the invention, one uses a single, common antenna for both resonant circuits, i.e. both for the high-frequency and for the low-frequency resonant circuit of the terminal. This considerably reduces the cost of providing the circuit and the space it occupies. It is further advantageous that there is no longer any mutual influencing of several antennas.
A coupling inductance coil is provided according to the invention in the resonant circuit for low carrier frequency between its capacitor and the antenna. The coil forms a high A.C. impedance for high carrier frequency but a small one for low carrier frequency. Consequently, only the resonant circuit for high carrier frequency communicates with data carriers designed for high carrier frequency.
The ratio of the A.C. impedence of the coil corresponds exactly to the carrier frequency ratio of the two resonant circuits (XL=2xcfx80xc2x7fxc2x7L). That is, the impedence of the coil is e.g. about 110 times greater at a high carrier frequency of 13.56 MHz compared to a low carrier frequency of 125 kHz. In general, the ratio of high carrier frequency to low carrier frequency should be at least 10, preferably at least 50.
In order to guarantee that only the resonant circuit for low carrier frequency communicates with data carriers designed for low carrier frequency, at least one capacitor is preferably provided between the transceiver electronics of the terminal and the resonant circuit for high carrier frequency. This capacitor forms, together with the capacitor in the resonant circuit for high carrier frequency, a high resistance for low carrier frequency but a low resistance for high carrier frequency. Consequently, only the resonant circuit for low carrier frequency communicates with data carriers designed for low carrier frequency.
At the same time, the capacitor (C2) between the transceiver electronics and the resonant circuit for high carrier frequency forms together with the resonant circuit capacitor (C1) an adapter circuit for the driver stage, thereby avoiding reflection and parasitic power radiation. The transceiver electronics of the terminal is connected with the antenna with a coax cable with a corresponding characteristic impedance.
Preferably, the resonant circuit for high carrier frequency has at least one shunt resistor in order to permit adjustment of the quality of said resonant circuit.
According to the invention, the two resonant circuits can be activated for communication alternatingly with a multiplexer for example. It is even also possible to activate both resonant circuits for communication simultaneously.